A hologram is a recording of an optical interference pattern between light waves. To generate a hologram, two coherent light beams from the one light source—called the object and reference beams—are made to overlap in a photosensitive material such as a photopolymer or silver-halide emulsion. The object beam propagates from the object and thus carries information about it, while the reference beam is used to both record and read-out the hologram. The optical interference pattern is physically stored as a change in absorption, refractive index or thickness of the recording material—turning it into a series of interference fringes (i.e. a diffraction grating) that contains information about the amplitude and phase of the two original light beams. By illuminating the grating with a reference beam to imitate the original reference beam, a copy of the original object can be reconstructed.
Holograms are widely used in many commercial applications including display holography, security, advertising and holographic optical elements and gratings.
The two most commercially important types of holograms are transmission holograms and reflection holograms.
A transmission hologram is one where the emergent rays leave the holographic support medium via the surface opposite that by which incident rays enter. The fringes of a transmission hologram are usually inclined to the surface at a considerable angle, for example, typically around 90°. The fringes of a transmission hologram run perpendicularly to the plane of the support medium and the spacing between fringes will remain virtually unchanged as the support medium swells or contracts. These holograms, which usually consist of a pattern on the surface of a plastic film coated on a reflective aluminium foil, are those used as security features on credit cards and other products—the foil allows the light to come from behind the hologram to reconstruct the image.
A reflection or Denisyuk hologram is a hologram where the interference pattern or grating is constructed by the reference and object beams entering the recording material from opposite sides. The holographic image can then be replayed by a point source of white light to imitate the original reference beam, thereby illuminating the hologram from the same side as the viewer. The fringes of a Denisyuk hologram are inside of and run substantially parallel to the surface of the holographic support medium. A Denisyuk hologram can reproduce a range of colours using white-light illumination on the same side of the hologram as the viewer.
Denisyuk or reflection holograms have traditionally been referred to as “volume gratings” as the fringe pattern is all located inside of the recording material. However, the term “volume gratings” is now also used to describe transmission hologram gratings where the interference pattern of fringes is recorded inside of the material, as well as on the surface of the material, as the two interfering beams overlap on the recording material from the same side. Usually only the surface patterns of transmission gratings are of interest commercially whereas with true reflection or Denisyuk holograms there is no surface grating.
As a Denisyuk hologram is produced by two beams hitting the support medium from opposite sides producing fringes that run substantially parallel to the plane of the support medium, like the pages in a closed book, the fringe spacing in a Denisyuk hologram will increase or decrease as the support medium swells or contracts. A change in fringe spacing means that when a replay of a hologram is made under a white light source then the reflection or Denisyuk-type hologram has the ability to vary its selection of wavelengths from the white light source thus giving a quasi-monochromatic replay colour to the image depending on the degree of swelling or contraction of the support medium. Reflection or Denisyuk-type holograms have found utility as sensors on the basis that interaction of an analyte or species to be detected with the support medium may cause a detectable change in the fringe spacing of the hologram.
In contrast, and as touched upon above, as the fringes of a transmission hologram run perpendicularly to the plane of the material, the spacing between fringes will remain virtually unchanged as the support medium swells or contracts. Accordingly, a transmission hologram will usually replay a rainbow of colours from a white light source regardless of the state of swelling of the support medium. For such reasons, transmission holograms have not found utility as sensors, rather they have found greater utility in data storage and security applications.
Typically, reflection or Denisyuk-type holograms use photographic emulsions made up of a polymer, a recording material such as a light-sensitive silver salt (silver bromide) and a photosensitizing dye coated onto glass or plastic substrates (holographic recording plate), and are constructed by passing a laser through the emulsion and returning it via reflection off a planar mirror that serves as the object. The standing-wave pattern created when the incident and reflected beams meet is preserved in 3-D via layers of ultra-fine grains of metallic silver, which means the hologram relies on diffractive reflection from the silver grains. To ensure that reflections from different locations interfere constructively with one other, the silver fringes must be spaced periodically.
Following exposure of the holographic recording plate, the plate must be developed, typically in a darkroom, in order to view the hologram. The developing process is long and tedious and typically involves the following steps:                a) Preparation of the developer and bleach solutions and, optionally, a wetting solution, and setting up appropriate apparatus' for use in the developing process;        b) Submersion of the holographic recording plate in the developer solution until the holographic recording plate has turned almost black;        c) Rinsing the developer solution from the holographic recording plate, typically using de-ionised water;        d) Submersion of the holographic recording plate in the bleach solution until the holographic recording plate is completely clear;        e) Rinsing the bleach solution from the holographic recording plate, typically using de-ionized water;        f) Optionally, placing the finished hologram in a wetting solution, which helps holograms to turn out much cleaner by reducing streaks and reducing drying time;        g) Drying the holographic recording plate.        
Up until approximately ten years ago, relatively few suitable holographic film materials existed, the most common type of holographic film being a silver halide-containing gelatin film made by a liquid phase colloid formation technique, followed by coating onto a suitable support layer.
WO-A-9526499 discloses a holographic sensor, based on a volume hologram. This sensor comprises an analyte-sensitive matrix having an optical transducing structure disposed throughout its volume. Because of this physical arrangement of the transducer, the optical signal generated by the sensor is very sensitive to volume changes or structural rearrangements taking place in the analyte-sensitive matrix as a result of interaction or reaction with the analyte.
WO-A-99/63408 discloses a method of production of a holographic sensor which utilises a sequential treatment technique wherein the polymer film is made first and sensitive halide particles are added subsequently. The particles were introduced by diffusing soluble salts into the polymer matrix where they react to form an insoluble light-sensitive precipitate and, thereafter, the holographic image is recorded. This method of production allowed holograms to be recorded in a much wider range of polymer matrices, for example, polyvinyl alcohol, polyacrylamides, polymethacrylamides, polyhdroxyethyl methacrylate etc., which had previously been considered to be unsuitable for use in holography.
Thereafter, it has been discovered that a wide range of both hydrophilic and hydrophobic polymeric materials are suitable for use in the production of transmission holograms. In contrast, while a wide range of hydrophilic polymeric materials have been found to be suitable in the production of reflection holograms, it has proved exceedingly difficult to successfully introduce holographic reflection gratings into hydrophobic polymeric materials, thus severely limiting the range of applications of reflection of Denisyuk-type reflection holograms. For example, polydimethylsiloxane (PDMS) is a hydrophobic polymer that possesses an extraordinary ability to swell in the presence of both liquid and/or gaseous low molecular weight hydrocarbons and organic solvents and thus is, theoretically, a particularly suitable candidate for use as the support medium in a holographic sensor. However, the techniques known to those skilled in the art of production of holographic sensors have not proved successful in introducing a Denisyuk reflection hologram into PDMS.
Laser-ablation techniques have been used to introduce a surface grating onto a support medium in the production of a transmission hologram (Yamada et al. Optics Express, 2002, Vol. 10, No. 21, pages 1173-1178 “Single femtosecond pulse holography using polymethyl methacrylate”). The term “laser ablation” is commonly understood to refer to the process of removing material from a solid (or occasionally liquid) surface by irradiating it with a laser beam. At low laser flux, the material is heated by the absorbed laser energy and evaporates or sublimates. At high laser flux, the material is typically converted to plasma. A femtosecond laser is used to fabricate transmission holographic gratings in transparent materials by two-photon absorption or multi-photon processes. The laser energy is typically focussed into a high energy spot of less than one millimeter. The interference pattern of fringes are recorded by removal of material from either the surface of the support medium or inside of the support medium, (thereby forming “volume gratings” as discussed above). As noted above, transmission holograms are fundamentally different to reflection holograms and, as is understood by those skilled in the art, techniques used to introduce diffraction gratings into a support medium to produce a transmission hologram are often unsuitable for introducing diffraction gratings into a support medium to produce a reflection hologram. In addition, a transmission hologram may, on occasion, be formed as an artefact on the surface of the support medium during the production of a Denisyuk hologram. These transmission holograms produce spurious grating effects of the surface of the support medium which adversely affect the clarity and hence applicability of the Denisyuk hologram.